A marble, a bead and a sticker costs $8.50 altogether.
The marble costs $2.10 less than the bead.
The sticker costs $2.50 more than the bead.
How much do 4 stickers cost?
Choose the cheapest item to be 1 u.
The marble costs $2.10 less than the bead.
The cheapest item is the marble.

Cost of the marble = 1 u
Cost of the bead = 1 u + 2.10
Cost of the sticker = 1 u + 2.10 + 2.50 = 1 u + 4.60

Total cost of the marble, the bead and the sticker
= 1 u + 1 u + 2.10 + 1 u + 4.60
= 1 u + 1 u + 1 u + 2.10 + 4.60
= 3 u + 6.7

3 u + 6.70 = 8.50
3 u = 8.50 - 6.70
3 u = 1.80

1 u = 1.80 ÷ 3 = 0.60

Cost of 4 stickers
= 4 x (1 u + 4.60)
= 4 x (0.60 + 4.60)
= 4 x 5.20
= $20.80

Answer(s): $20.80